The present invention relates to the art of diagnostic imaging. It finds particular application in conjunction with CT scanners for medical diagnostic purposes. However, it is to be appreciated that the invention will also find application in other operations in which an object is examined from multiple directions by an x-ray source.
Conventional CT scanners have an x-ray tube which is rotated around a patient disposed in a scan circle or examination region. Radiation detectors disposed opposite the scan circle from the radiation source convert the intensity of received radiation into corresponding electrical signals. In this manner, a measure of the radiation attenuation through the patient along each of a large multiplicity of known paths is determined. From this path and attenuation data, an image is reconstructed using conventional algorithms.
The prior art CT scanners included x-ray tubes and x-ray tube control circuits designed to have a constant, unwavering radiation output. To the detector receiving the radiation, variations in x-ray tube output appeared as variations in radiation attenuation in the subject. Frequently, a reference detector was provided to monitor for any fluctuations in radiation from the x-ray tube.
Commonly, the radiation detectors integrated the amount of received radiation between samplings. In many scanners, the sampling was triggered by the angular position of the x-ray source relative to the patient. Thus, any variation in the rotational speed of the x-ray source would cause a change in the amount of time between samplings, hence the amount of radiation integrated by the detectors. Again, this rotational speed error variation in the amount of received radiation appeared as variations in the radiation attenuation properties of the subject along the corresponding path and caused errors in the resultant image. Accordingly, the prior art CT scanners were commonly designed to optimize the uniformity of the x-ray tube rotation velocity.
One of the problems in this prior art CT scanning process is that the algorithms assumed that the examined subject was generally circular in cross-section with substantially the same radiation absorptive properties in all directions. In practice however, the human body is more often irregularly elliptical than circular. .Along certain paths, such as the major axis of the ellipse, there is generally substantially more radiation attenuation than along the minor axis. Because great care was taken to send the same amount of radiation along each path, the amount of radiation leaving the patient along the minor axis was much higher than the amount of radiation leaving the patient along the major axis. Commonly, the amount of radiation output by the x-ray tube was selected such that the range of radiation variation detected along the minor axis was in the upper part of the dynamic range of the detectors and variations in the range of radiation detected along the major axis was at the lower end of the dynamic range of the radiation detectors.
Because less radiation was detected along the major axis, the amplitude of the radiation relative to the noise was lower. Along the minor axis, the amplitude or amount of the received radiation was much higher relative to the background noise. That is, the noise statistics along the major axis were much greater than the noise statistics along the minor axis. This caused the resultant image to have higher noise statistics along the major axis than along the minor axis. This directionally dependent difference in noise statistics was commonly referenced as "structured noise" which may evidence itself in streaks that propagate along the thick or major axis direction. If the radiation attenuation along the major axis became sufficiently great, the detector response due to the incident radiation could become almost the same as the detector noise, a condition known as "photon starvation". Photon starvation resulted in streaks that are very pronounced.
Increasing the amount of radiation produced by the x-ray tube reduced photon starvation, but had adverse effects along the minor axis. Specifically, if too much radiation were received by the radiation detectors, the detectors saturated.
One technique used in the past to control does and detector saturation was shaped compensators. That is, because the radiation path through the patient along the center of the fan was generally longer than along oblique angles of the fan, a compensator was provided for reducing the amount of radiation in the edges of the fan relative to the center. This fan angle dependent dose reduction enabled more radiation to pass along the longer, central paths without saturating the detectors with the radiation that passed along the central paths.
An x-ray tube is confined to deliver a predefined amount of radiation during any given time period. This restriction in the amount of radiation both limits the x-ray dose to the patient and prevents damage to the x-ray tube. If the x-ray tube were driven to produce too many x-rays, the anode may be damaged. If the surface temperature reached a sufficiently high level, the anode could warp or even melt, changing the surface characteristics of the anode. To protect the expensive x-ray tubes, most CT scanners operate at a given tube voltage, at one of several selectable tube currents, and only for a limited amount of time.
For these reasons, the CT gantry and the x-ray tube are commonly designed to have a constant angular velocity and a constant radiation profile.
In accordance with the present invention, a new and improved CT scanner and scanning method are provided which overcomes the above-referenced problems and others.